In preparation for the first post-Columbia shuttle mission, two astronauts spent more than five hours in a neutral buoyancy tank here Feb. 24, practicing for one of three spacewalks they will conduct to repair the international space station and test new techniques for fixing heat resistant shuttle tiles.

The exercise by NASA astronaut Stephen Robinson and Soichi Noguchi, of the Japanese Aerospace Exploration Agency, was a test run of their planned extra-vehicular activity (EVA) — NASA vernacular for spacewalk — to replace one of the broken gyroscopes that is used to maintain the station’s orbit.

While the test went smoothly, officials here said more training is still needed before Noguchi and Robinson will be ready to test two methods to repair damaged shuttle tiles and cracks in the thermal protection panels made of reinforced carbon carbon (RCC).

“Most of the EVA you saw today is practicing and rehearsing stuff we’ve done before,” said James Kelly, who will be the pilot on the mission NASA has designated as STS-114. Kelly, who participated in the rehearsal from the spacewalk control room, said “we still have some things that are unknown, mostly things to do with tile repair and RCC repair.”

It was damage to the space shuttle Columbia’s RCC panels along a wing leading edge — sustained during launch when a chunk of foam broke off the orbiter’s external tank — that allowed hot gases to enter the wing and destroy the spacecraft during re-entry Feb. 1, 2003. Since the accident, researchers and engineers have been working to develop in-space methods to detect and repair such damage should it be necessary.

Veteran astronaut Eileen Collins, the commander of the STS-114 mission, told reporters that while she is fully confident that Discovery’s spaceflight will be the safest ever to fly, she does not believe that the tile and RCC repair techniques are mature enough to rely on should the flight require putting them in practice.

“Until we actually fly them in space and test them will they [the repair techniques] really be certified,” Collins said.

During the STS-114 spaceflight, Noguchi and Robinson are slated to test a black, putty-like substance that can be slathered onto cracks in RCC panels, as well as another technique called Emmitance Wash Application, where a gray substance is dabbed onto damaged tiles to shore up their heat protection qualities.

But Robinson agreed with his flight commander that those initial tests should not be pressed into action if repairs prove necessary aboard Discovery.

“We don’t want to have the first test be with us inside,” he said.

Other untested methods of tile and RCC repair may be on hand during Discovery’s mission, but will not be tested by the spacewalking duo, NASA officials said.

For the Feb. 24 spacewalk rehearsal — one of many for Noguchi and Robinson — the two astronauts donned training versions of U.S. spacesuits and entered the mock-space environment of NASA’s Neutral Buoyancy Laboratory — a vast pool containing large models of the space station and shuttle payload bay.

During the spacewalk rehearsal, Noguchi and Robinson practiced replacing a device called a control moment gyroscope used by the space station to maintain its position in space. The space station relies on four such gyroscopes for orientation, with two serving as spares. One of the devices — the one Noguchi and Robinson expect to replace — failed in 2002.

While the STS-114 spacewalkers had run through the gyroscope repair before, the Feb. 24 rehearsal was the first time they staged their EVA from Discovery’s airlock, rather than the space station airlock as originally planned.

“We’ve basically been learning more and more,” Robinson said. “It helps to have more things in our bag of tricks.”